1. Field of the Invention
The invention relates to the problem of monitoring the service life of filters.
2. Description of Related Art
Because of the limited fan power of many filter systems, particularly for filters used in air conditioning systems in motor vehicles, a drop in the volumetric flow rate occurs as dust accumulates on the filter. Therefore, it is desirable to be able to measure the degree of blockage of the filter in these systems in order to replace the filter in a timely manner.
A device for detecting the degree of blockage in a dust filter is known from German Patent Application 41 37 520 A1. Here, at least two sensors are situated in the air duct, each corresponding to different sections of the dust filter and sending electrical signals which indicate the air speed in the respective areas of the air duct. The difference in the air speed, which is measured by the sensors and compared in a potentiometer, shows the given degree of contamination in the dust filter. In order to obtain reliable results, various devices are installed in the flow channel to which sensors are associated to make a comparison with sensors installed at other locations. The device is thus sensitive and relatively costly to manufacture, and therefore is suitable only for larger filter systems. In addition, the sensor situated upstream from the filter may become contaminated.
Another possibility for monitoring the degree of contamination of a filter situated in an air stream is described in German Patent Application 41 01 739 A1. Here, a bypass line is provided parallel to the path of the air flow which bypasses the filter and in which a sensor in the form of a measuring sensor is situated which is responsive to changes in the flow speed and which actuates a display signal to a display device when a specific flow speed is exceeded. This device is also relatively costly, and, likewise, is therefore only practical for larger systems. The sensor is impinged on by unfiltered air and thus is not protected from contamination.
One air filtering option having a simple design and also being usable in air conditioning systems in motor vehicles is described in European Patent Application 0 847 787 A1. The filter used here is overlaid with a network of electrical conductors, the electrical resistance of the conductors being measured when air flows through the filter. The electrical resistance provides a measure of the flow rate. From this information, conclusions may be drawn as to the degree of blockage. The manufacturing costs are high, and assembly is resource-intensive.